This invention relates to a wire wound flexible shaft having an extended fatigue life, and a method for manufacturing the same.
Wire wound flexible shafts usually comprise a central or mandrel wire, upon which is wound successive helical wire layers, each layer normally being wound with a pitch direction opposite to that of the preceding layer. The shaft has a predetermined length dependent upon the desired use thereof. A flexible shaft assembly is made by providing a length of flexible shaft with fittings at the shaft ends which facilitate connection of one of the ends to a driving or shaft turning member such as a handle, crank or motor, and connection of the other end to a driven member such as a valve or a rotatable tool.
Such flexible shaft assemblies typically are used where the driving and driven members are not coaxial, so that while transmitting torque from the driving to the driven member the shaft must bend to accommodate the non-aligned positions of said members.
Unidirectional flexible shafts are intended for rotation and torque transmission in only one angular direction, while bidirectional shafts, the need for which arises less frequently, are intended to rotate and transmit torque in both angular directions.
In the large scale manufacture of such flexible shafts, a continuous section of shaft material is wound using winding machinery adapted for that purpose; and after the shaft material is wound, it is cut into the desired lengths, which are then provided with end fittings to complete the flexible shaft assembly.
When a flexible shaft transmits torque, the wire layers of the shaft rub against each other, and the resulting friction heats the shaft. Under some rotation conditions the wires of a flexible shaft can become red hot. This friction and heat cause the wires of the shaft to fatigue; and the shaft eventually breaks.
Attempts to increase the fatigue life of wire wound flexible shafts have focused primarily on providing more durable wire for making the shafts, and heat treatment to reduce internal stresses within the shaft. Attempts to reduce internal friction by use of lubricants have had limited success.
U.S. Pat. No. 5,288,270 to Ishikawa entitled Flexible Shaft Having Element Wire Groups And Lubricant Therebetween discloses a structure wherein grease is introduced into interstices of a wire wound flexible shaft in order to reduce friction. The lubrication approach taken by Ishikawa, however, has a number of drawbacks.
According to the teaching of Ishikawa the flexible shaft is made by winding a layer of hemp yarns around a central or core wire (also known as a mandrel wire). Successive layers of steel wire are then wound around the hemp layer. Thereafter the flexible shaft so formed is cut to a desired length and lubricated by vacuum impregnation with grease, in the course of which the grease penetrates into and is absorbed by the hemp yarn to form a grease-bearing layer. The grease also penetrates into interstices of the wires. The grease-bearing layer acts as a reservoir to slowly release grease into the flexible shaft interstices.
At low temperatures the grease in the flexible shaft of Ishikawa becomes more viscous, resulting in stiffening of the shaft accompanied by a decrease in flexibility and an increase in the torque required to start rotation of a driven member attached to the shaft assembly. The grease will break down in high temperature environments or in situations where a combination of environment temperature and residual internal friction sufficiently increases the temperature of the shaft wires. The grease tends to be axially pumped along the shaft as it rotates; and if the shaft is turned unidirectionally, as the majority of flexible shaft assemblies are, the grease would be pumped in one direction only and would accumulate at one end of the shaft while providing little or no lubrication at the other end. Cutting and handling of the grease impregnated shaft of Ishikawa would be extremely difficult, so that to manufacture the Ishikawa flexible shaft, the shaft material must be cut to desired lengths prior to impregnation, making the Ishikawa structure unsuitable for large scale manufacture.
The currently used method of lubrication is to apply grease to the outer wire layer of the flexible shaft. This method lubricates only the outer layer and does not reduced friction between the various wire layers of the shaft, as the lubricant does not reach the inner layers. High pressure lubrication of the outer layer has also been employed, with only limited improvement over the conventional outer layer lubrication method.
While each wire used to wind the flexible shaft is individually lubricated during spooling (i.e. when the wire is wound unto a spool for use in a winding machine), the lubricant burns off when the wound shaft is heat treated (typically at a temperature in the range of 800 to 900° F.) to reduce internal stresses.
Accordingly, an object of the present invention is to provide a wire wound flexible shaft having extended fatigue life, and a method for manufacturing the same.